Optimized electron occupancy of solid-solution transition metals for suppressing the oxygen evolution of Li2MnO3

Abstract

Li-rich layered cathodes based on Li₂MnO₃ have exhibited extraordinary promise to satisfy the rapidly increasing demand for high-energy density Li-ion batteries. However, besides cationic redox, oxygen anionic redox contributing to extra charge transfer often suffers from local structural transformation through O–O bond formation, leading to partial irreversibility during discharge. Four typical transition metal (TM) substitution models Li₂Mn_0.75TM_0.25O₃ (TM = Nb, Mo, Ru and Rh) are selected to calculate reversible capacity and perform electronic structure analysis. Our calculations reveal that the inherent t_2g orbital of TMs around the Fermi level plays a key role in stabilizing the anionic lattice and operating cationic redox, which further favors anionic redox reversibility. An electron occupancy number N_t2g = 4 of the t_2g orbital in substitutional TM cations is identified as the optimal value to achieve a maximum reversible capacity from cationic and anionic redox. Experimentally, Li₂Mn_0.75Ru_0.25O₃ exhibits improved reversible capacity by 18.1% at 0.1C compared to Li₂MnO₃, confirming the suppression effect of Ru on the oxidation of lattice oxygen. The present study provides a new insight into developing high-capacity layered electrodes with good reversibility.